[franze]

> but I am only lead on the project, I don't manage people.

I think thats the main problem. They, the team - and probably you, as you are part of the team - need management.

Clear goals. Intensive training. Clear modus operandi. Clear consequences (pos/neg).

Additional you as a team lead see your real "Handlungsspielraum". As a not manager you might not be able to fire them, but you can define what to do, what not to do and how to do it.

As the old way of doing things did not work, over your vacation try something new. A new way of doing things. And then go on vacation and do t check your work email.